media: doc: pixfmt-rgb: Replace '-' with 'X' to denote padding

Using '-' to represent padding bits and bytes make text and tables more
difficult to read. Use 'x' and 'X' instead.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>